The invention relates to a device and a method for influencing the position of knots between the upper thread and the lower thread when sewing with a sewing machine.
When sewing a material and/or a textile or non-textile planar formation with a sewing machine the upper thread and the lower thread are connected to each other and/or interlooped or knotted at every point the sewing needle pierces the material. The term “sewing machine” here comprises particularly household sewing machines and overlock sewing machines. The position of every knot and/or every connection site of the upper thread and the lower thread in reference to the respectively allocated piercing site in the material is dependent on the thread tensions of the upper thread and the lower thread and/or their temporal progression during the knot formation. This particularly relates to the last phase of the knot formation, with the upper thread being stressed in the last section of the upwards motion of the sewing needle by a thread lever, which is arranged between the sewing needle and a thread tension regulator. The thread tension regulator generally comprises two thread tension disks, which can be compressed against each other with an adjustable force, with the upper thread being guided therebetween. By the increased thread tension of the upper thread, on the one hand, the knot is tightened and, on the other hand, a defined quantity of the upper thread is pulled off the bobbin. When the thread tension and/or the tensile force of the upper thread changes in reference to the one of the lower thread or vice versa, the position of the knot also changes at the respective piercing site in reference to a neutral central position within the material. The greater the tension of the upper thread the farther away is the location of the knot from the central position in the direction of the top of the material. Inversely, the knot can be pulled towards the bottom of the material by reducing the tension of the upper tread in an analogous fashion, or can even come to rest at the bottom of the material and be displaced towards the adjacent piercing site of the sewing needle.
It is known to manually set a suitable compression force at the thread tension regulator using adjustment elements at the sewing machine, which force adjusted to the respective friction coefficient of the thread used in reference to the surface of the thread tension disks, so that the knots are pulled to the center of the material to the extent possible, at least when sewing in straight stitches and/or without any zigzag motions of the needle bar.
A thread tension regulator is known from DE10304780, in which the thread tensioning device and/or the pressure applied by the thread tensioning device upon the thread is adjustable via an electromagnet perpendicular in reference to the pressure applied. Via an operating device the target value may be predetermined for the desired pressure. The relationship between the driving current of the electromagnet and the resulting pressure is known from a reference sewing machine. This relationship and/or function may however deviate from the known reference relationship in other sewing machines. Accordingly, a correction value is determined and saved for every sewing machine, by which either the predetermined target value for the pressure of the thread tension regulator or the value of the driving current for the electromagnet can be corrected such that the resulting pressure of the thread tension regulator is equivalent to the target value set, independent from the respective sewing machine.
Due to the fact that the actual position of the knot during the sewing process is generally dependent on at least one or more parameters, such as type and thickness of the material, friction coefficient, and the tension of the lower thread as well as the upper thread, type of knot, pivotal position of the needle bar, length and width of the stitch, and/or amount, direction, and speed of the change of the stitching position between successive sewing stitches, type of sewing machine, environmental conditions, such as humidity and temperature and the like, the results of a fixed predetermined setting of the thread tension regulator is frequently not satisfactory. Particularly in case of zigzag or buttonhole seams, the dependency of the insertion of the knot from the pivotal position of the needle bar may lead to the knots being pulled on one side optimally into the center of the material at the piercing sites, however those on the other side are not.